This invention relates to the field of electromechanical actuators and, particularly, to shape memory effect actuators.
The use of shape memory alloys in actuators of all types is well known. These actuators typically take advantage of the shape memory effect to trip a switch or break a contact upon reaching a critical temperature. For example, in U.S. Pat. No. 3,707,694 to DuRocher, a shape memory alloy element is heat treated to form a first shape which is a substantially planar configuration. Once below the alloy's critical temperature, the element may be deformed into a second shape which is arcuate. When the critical temperature is reached during operation, the shape memory alloy will "remember" its first shape and conform, or attempt to conform, to it. In DuRocher, there is a biasing member which deforms the shape memory alloy element into the arcuate shape so as to make an electrical contact when below the critical temperature. Upon reaching the critical temperature, the shape memory alloy element returns (or attempts to return) to its planar configuration, displaces the biasing member, and breaks the electrical contact. The heat supplied to the actuator may be in the form of electrical resistance or an outside heat source.
Other examples of actuators are disclosed in Willson U.S. Pat. No. 3,594,674; Willson, U.S. Pat. No. 3,594,675; Willson et al., U.S. Pat. No. 3,613,732; Willson et al., U.S. Pat. No. 3,634,803; Hickling, U.S. Pat. No. 3,849,756; Hart, U.K. Patent Specification No. 1,554,738; and Hart, U.K. Pat. Application No. 2,024,422A.
Some of these actuators will automatically reset upon cooling down from above the critical temperature. This is the case of the above-mentioned DuRocher patent where the biasing member, once below the critical temperature, deforms the shape memory alloy element back into electrical contact with a substrate, thereby resetting the actuator.
Others of these actuators, such as in the Hickling U.S. Patent and the Hart U.K. patent application need to be manually reset. That is, the shape memory alloy element needs to be manually deformed into contact with a cooperating member so that it will once again be capable of activating upon exposure to a heat source.
The shape memory alloys that have been used with the above actuators are usually either copper-based or nickel/titanium based. These alloys are well known to those skilled in the art.
Whether the actuator is manually or automatically reset, a common element of all of the above actuators is that the actuator is activated upon reaching the critical temperature which is normally above room temperature. Under certain circumstances, however, it would be desirable to manually position the actuator (without heating it) prior to reaching the critical temperature while also having it automatically resetable.
To manually position the actuator prior to reaching the critical temperature is difficult to achieve since the biasing member is designed to be stronger than the martensitic shape memory alloy element. Thus, any attempt to manually actuate the actuator by movement of the weaker shape memory alloy element will be overcome by the biasing member.
Accordingly, it is an object of this invention to be able to manually position a shape memory effect actuator as well as activate it in response to a change in temperature.
It is another object of this invention to have a shape memory effect actuator which is relatively simple in design yet is capable of being manually positioned as well as activated in response to a change in temperature.
These and other objects of the invention will become apparent to those skilled in the art after reference to the following specification considered in conjunction with the accompanying drawings.